Introduce a field trial for Instant.

chrome/common/pref_names.cc

Index: chrome/common/pref_names.cc
diff --git a/chrome/common/pref_names.cc b/chrome/common/pref_names.cc
index 22eb9214dcdaf31c79b2cbb506e2ed3aa2f4ac0d..d778c2bca99489da6478571f2130afa1c0a9a2c6 100644
--- a/chrome/common/pref_names.cc
+++ b/chrome/common/pref_names.cc
@@ -275,6 +275,10 @@ const char kInstantEnabledOnce[] = "instant.enabled_once";
 // Time when instant was last enabled.
 const char kInstantEnabledTime[] = "instant.enabled_time";
 
+// Random number drawn and used by the Instant field trial. Stored in prefs to
+// provide a consistent field trial experience across browser restarts.
+const char kInstantFieldTrialRandomDraw[] = "instant.field_trial_random_draw";
+
 // Used to maintain instant promo keys. See PromoCounter for details of subkeys
 // that are used.
 const char kInstantPromo[] = "instant.promo";